It is a problem in the field of atmospheric meteorological and other parameter measurement to passively, remotely, inexpensively, and continuously measure these parameters. Active remote sensing systems emit often objectionable or unacceptable radio or light energy and are power consumptive. Front-end, operating, and maintenance and repair costs can be high.
Every substance that is above absolute zero in temperature emits radiation across the entire electromagnetic spectrum in accord with the quantum mechanical Planck function defined in Equation 1 below.
                              I          ⁡                      (                          λ              ,              T                        )                          =                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          hc              2                                                          λ              5                        ⁡                          [                                                ⅇ                                                            hc                      /                      λ                                        ⁢                                                                                  ⁢                    kT                                                  -                1                            ]                                                          (        1        )                            I is radiated power per wavelength interval,        λ is the wavelength of the radiation        h=Planck's constant,        k is Boltzmann's constant,        T is temperature,        c is speed of light.        
The residual radiation temperature of outer space from the Big Bang is about 3 Kelvins, or minus 270 degrees Celsius. The gases in a cloudless sky emit radiation proportional to their physical temperature and add to downwelling 3 Kelvin cosmic radiation, yielding a temperature as sensed in the 8 to 14 micron window region as cold as minus 100 Celsius or colder. If significant water vapor or aerosols or dust are present in a cloudless sky, this infrared temperature can be warmed to minus 50 Celsius or warmer. Due to absorption and scattering by water droplets, infrared radiation has a short photon path length in clouds, and therefore emanates from the cloud margins. Thus the infrared temperature seen of a cloud is simply the local physical temperature of the edge of the cloud, plus a small contribution to temperature due to the absorption and emission of the intervening atmosphere. By knowing or estimating the temperature profile of the atmosphere and measuring the cloud temperature, one can determine the altitude and range of the cloud.
This is true for water in liquid or ice phase. It is also true for hydrometeors such as rain and snow that the infrared radiation path length is short and therefore radiation from the margins of precipitation volumes reflects the local physical temperature.
By remotely measuring the above temperatures across regions of the sky, much can be determined regarding the present content and meteorological state, evolution, and future state of said state and content of the atmosphere. Because the cloudless atmosphere is somewhat transparent in the 8 to 14 micron region of the spectrum, observations of meteorological phenomena such as clouds and precipitation in this spectral region are preferred.
Many measurement systems that characterize parameters of the atmosphere have negative aspects and attributes. Radars, LIDARs, and other atmospheric sensor systems are power consumptive, are costly, and often are comprised of expensive finite lifetime components. Operational costs can therefore be high. Operational weather radars also cannot detect and track very small cloud droplets because the reflected signal is roughly proportional to the 6th power of the droplet diameter. Radiation emitted by active remote sensing systems is sometimes intrusive, hazardous to health, and in some military and like applications, compromises covertness. It is therefore an object and characteristic of the present invention to be very low power, on the order of several watts, capable of fast sampling rates, inexpensive to purchase, implement, and operate, and be completely passive and safe in that it emits no radiation signal. Stand-alone, the invention can inexpensively yield valuable information relating to the state and content of the atmosphere.
Additionally, some atmospheric sensor systems are functional only under certain atmospheric conditions. Radars return signal only when significant clouds are present, LIDARs and Fourier Transform Interferometers (FTIRs) do not operate through cloud. It is therefore an object of the present invention to yield signal that can command the commencement and cessation of operations of other such sensing systems based on sky conditions, thereby creating cost savings.
The present low cost invention is suitable for autonomous operation at remote unattended sites to observe present weather, visibility, and other atmospheric content and conditions and to forecast said parameters. For instance, the invention could report sky conditions and ceiling height at unattended airports to incoming air traffic. It can also detect and report fog, cloud structure and type (e.g., stratus, cumulus, cirrus), define sky coverage types (scattered, broken, fraction, quadrants occupied, and persistence of cloud cover). It can additionally report frontal passages, and give advance notice and prediction of the time of frontal passage. It can also characterize precipitation type and intensity. Because it senses thermal emissions, it operates equally well in daylight and darkness.
Infrared noncontact thermometers have heretofore been utilized in a zenithal mode to simply measure cloud base temperature, but heretofore not in the surveillance mode and not with the interpretative and predictive capabilities of the present invention.